An integrated circuit (IC) is a set of electronic circuits that are manufactured onto a semiconductor, notably silicon. ICs can be made very compact, having upwards of 10 million transistors or other electronic components per mm2 and growing. As such the width and size of the conducting lines and interconnections used to connect the transistors and other components to the rest of the microcircuit need to be made smaller and smaller as the technology advances, currently tens of nanometers.
In some cases, when an IC, or microchip, is manufactured, electrical connections called leads are attached from the microchip to a package in which the chip resides using wire bonds. The package is then bonded to a circuit board using a number of techniques such as, J-wing, gull-wing and solder bump interconnects. Many layers of materials, both conductive and non-conductive, are required to interconnect the transistors of microchips to packages to circuit boards and the outside world. In these constructions, distribution layers are necessary.
A redistribution layer is an extra layer of wiring on the chip and/or package that enables electronic interconnection of the microchip to other microchips, to the package and/or to the circuit board, and can be many layers of varying thicknesses, and resolutions. Redistribution layers are also used in chip stacking technologies.
For example, redistributing IC bond pads before flip chip bumping has become a common process for interconnection. Bumping is a process using solder which is applied as a solder paste and reflowed to create a round ball, or bump, of solder. Redistribution layers allow solder paste bumping of die originally designed for wire bonding. Wire bonding of the IC provides connections only to the one dimensional periphery of the IC, while redistribution layers and bumping allows the connections to be distributed throughout the two dimensional surface of the IC. While stud bumps and plated bumps could tolerate the small size and close spacing (for example 100 micron square pads on 150 micron pitch) of wire bond pads, solder paste generally requires more than twice that spacing. Converting peripheral wire bond pads to an area array of solder-bump pads by redistribution overcomes that barrier.
In some applications, redistribution offers an attractive method to create distributed power and ground contacts. Redistributed pads also transform off-chip connections from chip scale to board scale, as an alternative to expensive multilayer substrates. Wafer-level chip-scale packages often redistribute to ball-grid array pads as their final external package connection.
More compelling needs have been driving redistribution. Advances in chip scale packaging, wafer-level packaging, and most recently, 3-D packaging and system-in-package, often require redistributed bond pads.
A redistribution layer consists of wiring for electronic interconnect, processed by electroplating, vapor deposition, electroless plating or combinations thereof. Redistribution layers also require materials with low dielectric properties to isolate and insulate the interconnect wiring. As chip and consequently packaging and redistribution layers are pushed to be smaller and smaller, the material properties need to be able to continue to insulate and isolate the interconnects. Also as the dimensions decrease, processing methods needs to address ease of manufacture, cost issues, repeatability, and control.
In many instances polyimide is being used as in distribution layers, as well as organosilicon, benzocyclobutane and other exotic and expensive materials, that each have their own level of complexity, both in synthesis and in processing.
Thus there is a need for improved materials and improved processes that are designed to meet the new and ever demanding integrated circuit technology, particularly in the area of redistribution.